The present invention relates generally to current shunt amplifiers, and more particularly to improving the common mode input range thereof well beyond the power supply voltage.
Referring to Prior Art FIG. 1, the illustrated current shunt amplifier 1A measures the voltage produced across an external shunt resistor 5. Current shunt amplifier 1A includes an operational amplifier 2 having its (−) input connected by a conductor 8 to one terminal of an input resistor 3 and to the drain of a N-channel output transistor 7. The other terminal of input resistor 3 is connected by conductor 18A to one terminal of shunt resistor 5. The (+) input of amplifier 2 is connected by conductor 13 to an terminal of input resistor 4 and to the drain of a N-channel output transistor 12. Another terminal of input resistor 4 is connected by conductor 18B to the other terminal of shunt resistor 5. The gate of output transistor 7 is connected by conductor 6 to a first output out1 of amplifier 2 and the gate of output transistor 12 is connected by conductor 11 to a second output out2 of amplifier 2. The source of output transistor 7 is connected by conductor 9A to the (−) input of an output amplifier 10 and to one terminal of a source follower resistor 15, the other terminal of which is connected to a reference voltage Vref. The source of output transistor 12 is connected by conductor 9B to the (+) input of output amplifier 10 and to one terminal of a source follower resistor 14, the other terminal of which is connected by conductor 16 to reference voltage Vref. Output amplifier 10 can be a programmable gain amplifier or an instrumentation amplifier.
Current shunt amplifier 1A typically uses an emitter-coupled or source-coupled pair of input transistors to amplify the differential input voltage across shunt resistor 5. This topology needs four operational modes (two directions of the shunt current ISHUNT, each direction being associated with a high common mode input voltage and a low common-mode input voltage) in order to accommodate all possible input conditions and to meet requirements such as bidirectional shunt current flow and wide common mode range. The output signal Vout on conductor 17 requires programmable gain amplifier (PGA) 10 to have a very low common mode input voltage. These requirements necessitate an undesirably large die size for implementation of current shunt amplifier 1A. Another drawback of current shunt amplifier 1A is that its circuitry does not function well when the differential input voltage across shunt resistor 5 is near 0 volts.
In Prior Art FIG. 2, the illustrated current shunt amplifier 1B measures the voltage produced across external shunt resistor 5. Current shunt amplifier 1B includes a front end stage including an operational amplifier 30, input resistors 32A and 32B, and feedback resistors 34A and 34B connected as shown. The output stage includes an instrumentation amplifier 31 having its (+) input connected to the output 35 of the foregoing front end stage and its (−) input coupled by a gain resistor 37 to a reference voltage Vref and by feedback resistor 39 to Vout. The front end stage provides a fractional gain (i.e., a gain much less than 1 (e.g., 0.02)) to enable it to have a wide common mode input voltage range. Instrumentation amplifier 31 has a very large gain. Then, if shunt resistor 5 is at a very high voltage, e.g. 200 volts, which is far beyond the operational range of operational amplifier 30, large values of input resistors 32A and 32B can reduce the voltages applied to the (−) and (+) inputs of operational amplifier 30, greatly reducing its gain. However, a high compensating gain can be provided by instrumentation amplifier 31. Unfortunately, current shunt amplifier 1B of Prior Art FIG. 2 has the drawbacks of high noise and large die size.
Prior Art FIG. 3 shows a simple form of an instrumentation amplifier 1C having essentially the same topology as that of the front end stage of Amplifier 1B shown in Prior Art FIG. 2. Instrumentation amplifier 1C is connected to measure the voltage produced across external shunt resistor 5. The topology of instrumentation amplifier 1C has two major problems, the first being that the CMRR (common mode rejection ratio) is heavily dependent on the matching of the various resistor ratios. The second problem is that the common mode input range (CMIR) is limited by VDD, as is also the case for the shunt amplifiers of FIGS. 1 and 2. The CMIR is from ground to VDD.
Thus, there is an unmet need for a current shunt amplifier having a common mode input voltage range which extends substantially beyond the power supply voltage.
There also is an unmet need for a current shunt amplifier having a common mode input voltage range which extends substantially beyond the power supply voltage and which also is capable of bidirectional input current shunt measurements.
There also is an unmet need for a current shunt amplifier having a common mode input voltage range which extends substantially beyond the power supply voltage and which also is completely operational when the common mode input voltage is at zero or ground volts.
There also is an unmet need for a single stage current shunt amplifier having a common mode input voltage range which extends substantially beyond the power supply voltage.
There also is an unmet need for a current shunt amplifier having a common mode input voltage range which extends substantially beyond the power supply voltage and which also provides high accuracy current measurement without trimming of circuit components such as resistors.